Grand Prix "Runs away"

keepledudekeepledude Member Posts: 2
edited July 2015 in Pontiac
My 2002 Grand Prix GTP has 185000km on it and has been great to me except for this one(major) issue that started not too long ago. The car, at times, will just rev up on its own for no apparent reason to anywhere from 2000-4500 rpm and just sticks there for some time or untill the car is shut off. If it is shut off in the middle of one of its fits, it will not start unless the gas peddle is held to the floor, and when it does start it blows smoke and sounds like its only running on 3 cylinders and it resumes running away once its firing on all 6 again. It will run away when its hot, cold, at speed, stoped and completely at random. I have looked for a cause to its running away and replaced the MAF, IAC and TPS with brand new ones and it still does this. Have disconected the cruise control because I have heard that that maybe be a cause and it still runs away even after doing so. Only way I have found to stop it running away is to just wait for it to stop which usualy takes a few minutes(5-6) of it just sitting in park or floor it through first gear and let it shift once and that will usualy bring it back down to what it normaly should be at and it will carry on like nothing happened untill it acts up again. Pretty sure it is not a vacum leak because it does stop and its not constant. I am a bit loss with this issue and can't think of what else may be causeing it. Any ideas?


  • thecardoc3thecardoc3 Member Posts: 5,557
    The computer can only increase the idle speed to the maximum that the idle air control motor is capable of. To diagnose this a tech would connect a scan tool and start by looking at the idle air control data. Either the computer is commanding the high idle (high IAC counts in the data) or else it is unable to control the idle speed (low counts).

    If the counts are low then the computer is trying to correct for what-ever has the idle speed increased. The odds are however that the counts are going to be high since this is random and that would mean that the computer is commanding that high idle speed when it occurs. The tech would be able to monitor inputs like the AC clutch command, power steering pressure switch and even the charging system voltage as seen by the PCM (among some others) and see if something is out of range and then confirm if the data matches the real input. The power steering pressure switch input for example causes the computer to increase the idle speed in order to prevent stalling during high load conditions caused by the power steering. If it falsely see's a power steering load, then the result is usually a very high idle speed.

    The biggest this is stop throwing parts, someone has to test and prove what is going on. No-one should just shoot from the hip and throw a shopping cart load of parts at a problem. There isn't a more expensive way to try and repair a car than to blindly throw parts at it.
  • keepledudekeepledude Member Posts: 2
    The MAF and TPS had gone screwy and were replaced to deal with the sudden stalling issue that it had developed before this. The IAC was done because thats what was thaought what was causing it. I will hopefully be able to plug in our scanner and see if it will act up when pluged in for once.
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